(a) Technical Field
The present invention relates to a cathode material for a secondary battery, and a method of manufacturing the same. More particularly, it relates to a cathode material for a secondary battery, and a method of manufacturing the same, wherein a lithium manganese phosphate LiMnPO4/sodium manganese fluorophosphate Na2MnPO4F composite can be used as an electrode material.
(b) Background Art
As the use of portable small-sized electronic devices has increased, there has been rapid progress in the development of new types of secondary batteries such as nickel metal hydrogen or lithium secondary batteries. In particular, the lithium secondary battery uses carbon (such as, e.g., graphite) as an anode active material, and lithium-containing oxide as a cathode active material, while using a non-aqueous solvent as an electrolyte. Lithium is a metal that has a very high ionization tendency, and thus can achieve a high voltage. Consequently, lithium is used in the development of batteries having a high energy density.
A lithium transition metal oxide containing lithium is mainly used as the cathode active material, and also, 90% or more of the lithium transition metal oxide used for the cathode active material includes layered lithium transition metal oxides (such as, e.g., cobalt-based, nickel-based, and cobalt/nickel/manganese ternary-based). However, when layered lithium transition metal oxide mainly is used as a cathode active material, lattice oxygen may be deintercalated and made to participate in a reaction under a non-ideal state (overcharge and high temperature). Disadvantageously, this may cause significant problems, such as the battery catching fire.
In order to overcome these disadvantages, research on a cathode active material having a spinel or olivine structure has been conducted. As a means for solving the problems with lithium secondary batteries such as, for example, the lowering of stability due to cathode deterioration, it has been suggested that spinel-based lithium manganese oxide having a three dimensional lithium movement path, and polyanion-based lithium metal phosphate including an olivine structure, may be used as a cathode material instead of layered lithium transition metal oxide. Unfortunately, the use of spinel-based lithium manganese oxide is limited because it causes lithium elution during charging/discharging cycles, which causes structural instability due to the Jahn-Teller distortion effect.
With respect to the olivine-based lithium metal phosphates, iron (Fe)-based phosphate and manganese (Mn)-based phosphate have low electric conductivity, which greatly limits their use as a cathode material. However, through nano-sized particles and carbon coating, the problem was improved, and thus their use as a cathode material has become possible.
It has been recently been reported that fluorophosphate (containing fluorine) may be used as a polyanion-based material. Fluorophosphate has a formula including fluorine, for example, A2MPO4F, where A represents Li or Na, and M represents a transition metal such as Mn, Fe, Co, Ni, V, or a mixture thereof. Theoretically, fluorophosphate is expected to show a capacity about twice as high as a conventional lithium metal phosphate since it has two Na atoms. Also, in the case where Na2MPO4F (M=Mn, Fe, Co, Ni, V, or a mixture thereof) is used as a cathode material for a lithium secondary battery, the sodium is deintercalated during an initial charging step, lithium is intercalated during an initial discharging step, and then in following cycles, intercalation/deintercalation of lithium is carried out during charging/discharging. Also, in the case where it is used as a cathode material for a sodium secondary battery, the intercalation/deintercalation of sodium is carried out during charging/discharging. Disadvantageously, iron (Fe)-based LiFePO4 and Na2FePO4F have a low charge/discharge potential (about 3.5 V). A further disadvantage is that these cathode materials are typically synthesized through a complicated process of mixing by ball-milling, which increases their cost.
Accordingly, there is a need in the art for a low cost cathode material with excellent charge/discharge properties.